Leave function ends to #6821's zero return

The text rewrite appended `return T(0);` to every value-returning
function, which doesn't compile for a struct type (`S(0)`). Main's #6821
gives naga's added end returns the result type's ZeroValue, structs
included; this PR's early-return test passes on it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-shaders/src/translate/rewrite.rs

@@ -237,11 +237,6 @@ impl Stage<'_> {
                     depth -= 1;
                     hidden.retain(|&(_, _, d)| d <= depth);
                     if depth == 0 {
-                        if let Some(ty) = returns {
-                            out.push_str(" return ");
-                            out.push_str(ty);
-                            out.push_str("(0); ");
-                        }
                         params.clear();
                         hidden.clear();
                         returns = None;